1. Field of the Invention
The invention relates to Orthogonal Frequency Division Multiplexing (OFDM) techniques, and, more specifically, relates to a method and a device for channel estimation in an OFDM system.
2. Background of the Invention
The OFDM technique is one of the frequency division multiplexing techniques for high rate data transmission services. Compared with traditional single carrier techniques, the OFDM provides higher spectrum efficiency by using simple equalization algorithm. Meanwhile, it is unnecessary to allocate frequency guard bands between adjacent subcarriers to avoid frequency interferences, which is adopted in the traditional Frequency Division Multiplexing (FDM), so that the bandwidth is saved.
Recently, the OFDM technique is widely used in the communications system, and has been applied in the wireless LAN standard 802.11a and the fixed wireless access standard 802.16a. Besides, regarding the wireless access network of 3GPP and the physical layer of IEEE 802.20, the OFDM technique is being considered for constructing mobile wireless access system with higher spectrum efficiency.
FIG. 1 shows a networking diagram of a typical frequency multiplexing cellular system. In this system, two Radio Network Controllers (RNC), RNC1 and RNC2, are connected to the Core Network (CN); some Base Stations (BS) are connected to one of the two RNCs: BS1, BS2 and BS3 connecting to RNC1, while BS4, BS5 and BS6 connecting to RNC2; two Mobile Stations (MS), MS1 and MS2, keep wireless connections with these BSs. FIG. 2 is a typical cell omnidirectional antenna multiplexing mode, or succinctly called cell multiplexing mode. FIG. 3 is a typical cell 120 degrees directional antenna multiplexing mode, or briefly called sectored multiplexing mode. A data transmission system using OFDM technique has the following advantages:
1. Good robustness under multipath delay spread. As shown in FIG. 4, a time-domain OFDM symbol includes two parts: a data part and a cyclic prefix part; the cyclic prefix part is generated by circulating the last portion of the data part; as shown in the diagram the data part occupies a duration Tdata and the cyclic prefix part occupies a duration Tcp. The robustness of the OFDM technique refers to that: comparing with an OFDM symbol duration Ts, a typical channel impulse response duration is quite short and occupies only a quite short part of the Ts, so by increasing a shorter cyclic prefix, i.e. the Tcp, the interference between signals caused by multipath is completely eliminated.
2. Good robustness under frequency selective fading. Through redundant solutions such as channel coding, the OFDM technique can recover the digital signal carried by a badly fading subcarrier.
3. Simple equalization algorithm. The OFDM technique transmits the signal in the frequency domain and simple multiplications can be used to express channel effects in the frequency domain, so that a simple one tap equalizer could be used to equalize the signal in the OFDM system.
4. Higher spectrum efficiency comparing with the commonly used FDM technique.
Although the data transmission system utilizing OFDM technique has the above advantages, in order to realize the advantages in a practical system, and more importantly, to make the OFDM system work normally, it is necessary to develop several key technologies including: frequency synchronization, symbol synchronization, frame synchronization, channel estimation and equalization etc. These key technologies relate not only to application environment of the system but also to requirements on the network configuration of the system.
The purpose of the channel estimation in the above key technologies is that: through the channel estimation, the receiver can obtain the frequency domain information of the channel on which the transmitter transmits the data; based on the frequency domain information of the channel, the receiver can make equalization processing to obtain the data. Therefore, the channel estimation is an important precondition for the receiver to obtain the data correctly and efficiently.
IEEE 802.11a protocol provides a channel estimation technology. The frame structure of an 802.11a system is shown in FIG. 5. Every frame includes a preamble and a data OFDM symbol with uncertain length. The data OFDM symbol includes user data and signaling. The pilot subcarrier allocation solution of 802.11a is shown in FIG. 6. In the physical layer selection solution of 802.11a and 802.16a, the channel estimation uses the preamble. Specifically, the receiver knows the date carried in every subcarrier of the preamble transmitted by the transmitter, so that with the received preamble, the channel condition of every subcarrier of the preamble can be obtained; when the channel environment varies slowly, the channel condition of the subcarriers of the preamble could be considered as the channel condition of the corresponding subcarriers of the OFDM symbol.
In other words, the channel estimation solution provided by 802.11a is based on that channel condition of the data OFDM symbol is approximate to the channel condition of the corresponding preamble. When the channel environment varies rapidly, this approximation will make greater errors. Besides, the channel environment also varies due to the relative movement between the receiver and transmitter. So the above solution is not effective while being applied in the system with rapidly varying channel environment. The channel condition in the current mobile wireless communication system varies rapidly, so obviously the channel estimation solution provided by 802.11a is not suitable for the mobile wireless communication system.
Although in the 802.11a, pilot subcarriers are introduced to trace the variety of the channel so as to amend the channel condition of the subcarriers of the preamble, and to take the amended channel condition of the subcarriers of the preamble as the channel condition of the subcarriers of the data OFDM symbol, the amend can not reflect the rapid variety of the channel condition and will still cause a considerable degradation in the performance.
In order to solve the disadvantages mentioned above, a pilot subcarrier allocation solution of time-frequency grid is proposed by the art and shown in FIG. 7. In this solution, the pilot OFDM symbol, i.e. the preamble, is evenly distributed on the time-frequency plane. Utilizing the pilot OFDM symbol to trace the variety of the channel can partially solve the problem of varying channel.
The SIEMENS Corporation has proposed to the 3GPP RAN1 a proposal Tdoc R1-030780 about a specific solution for allocating the pilot frequency with the time-frequency grid mode, as well as the channel estimation method and the simulation result. The solution uses twice-order one-dimension interpolation to obtain the data subcarrier channel condition on the time-frequency plane: first making third-order Lagrange interpolation on the time-domain, and then making seventh-order Lagrange interpolation on the frequency-domain. The simulation result provided by SIEMENS shows that: comparing the ideal channel estimation, the channel estimation solution of SIEMENS has 0.5-0.7 dB performance degradation for the PA3, PB3 and VA30 channels and even has floor error at BLER=0.13 for the VB30 channel. This means that for channels with large delays, the channel estimation solution of SIEMENS makes large performance degradations.